UNITED NATIONS, Sept. 11 (Xinhua) -- UN Secretary-General Antonio Guterres said Tuesday that it is "absolutely essential" to avoid a full-scale attack on Syria's Idlib, the last major rebel stronghold in the country.

"This (A full-scale attack) would unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict," said the UN chief in a press encounter at UN Headquarters in New York.

Almost half of Idlib's population of 2.9 million people had gone there to seek refuge from conflict elsewhere in Syria, including nearly 1 million children. Their lives have been upended and they have nowhere to go, he said.

"Idlib is the last so-called 'de-escalation zone' in Syria. It must not be transformed into a blood bath."

Guterres reminded parties to the conflict of their obligations under international law to protect civilians.

"I understand that the present situation in Idlib is not sustainable and the presence of terrorist groups cannot be tolerated. But fighting terrorism does not absolve warring parties of their core obligations under international law," he said. "I make a clear appeal to all parties directly and indirectly involved -- and in particular the three guarantors of the de-escalation zone, namely Iran, Russia and Turkey: Spare no effort to find solutions that protect civilians. Preserve basic services such as hospitals. Ensure full respect for international humanitarian law."